Thermo-optic phase-shifting devices are essential components of optical communication systems. By thermally changing the refractive index of material in an optical pathway, they can control switching, attenuation or modulation of an optical signal. The principle of operation is that by heating a waveguide, the lightwave in the waveguide can be delayed enough to cause a change from constructive to destructive interference (or vice versa) with an undelayed lightwave, resulting in switching.
A typical thermo-optic phase shifter comprises a resistance heater thermally coupled to the high index core of a silica waveguide. Heat changes the temperature of the core and thereby the refractive index since it is temperature dependent. This changes the integrated product of index and distance (optical pathlength) and hence changes the time required for the passage of the light.
While such phase shifters are simple to fabricate and operate, they are unfortunately slow and consume too much power for many applications. Typically their switching frequencies are limited to a few kHz and they consume about 50-350 mW of electrical power. Phase shifters that could provide faster switching at comparable or lower power would be highly desirable.